The present invention relates to hydrodynamic transmissions in general, and more particularly to improvements in hydrodynamic reversing transmissions of the type employing one or more fluid flow power devices in each of the forward and reverse drive trains which connect the rotary input member of the transmission with the rotary output member.
Fluid flow power devices of the type known as torque converters can be made effective by filling them with a suitable working fluid (e.g., oil). The converter becomes ineffective as a result of draining, i.e., on evacuation of working fluid from its interior. Presently known hydrodynamic reversing transmissions comprise a pump or another suitable source of working fluid and a reversing valve which is interposed between the pressure line of the pump and the filling and draining connections of torque converters in the forward and reverse drives. The reversing valve is adjustable between two positions in one of which the filling line of the torque converter in the forward drive train is connected with the pump and the draining line of such torque converter is sealed. In the other position, the reversing valve connects the pump with the filling connection of the torque converter in the reverse drive train and simultaneously seals the draining connection of such torque converter. Transmissions of the just outlined character are often used in vehicles which must change the direction of travel at frequent intervals. Examples of such vehicles are diesel shunting locomotives. An important advantage of a hydrodynamic reversing transmission is that shifting into forward or reverse gear does not entail any wear upon the parts, i.e., the reversing valve merely causes the torque converter in the forward drive train to receive working fluid while the torque converter in the reverse drive train is drained, or vice versa. In fact, it is even possible to effect a changeover while the vehicle is in motion so that one of the drive trains furnishes a braking action.
A drawback of presently known hydrodynamic reversing transmissions (e.g., of the type disclosed in German Auslegeschrift No. 15 80 952) is that the interval which elapses during changeover from forward movement to movement in reverse is relatively long, i.e., the response of the vehicle to the commands of the driver is rather sluggish.
U.S. Pat. No. 3,347,113 granted Oct. 17, 1967 to C. A. Ramsel discloses a transmission which can be used in earth moving vehicles or for analogous purposes. Such vehicles, too, must move back and forth at frequent intervals, e.g., during transfer of earth, rock, ore or like flowable material from the ground onto the platform of a truck, railroad car or the like. The transmission of Ramsel employs a single torque converter coupled to a mechanical gear box which is switched under load. When changing gears, especially during frequent changing of the direction of movement, the wear upon the friction-type switching devices which are used in such transmissions is very pronounced. The same applies when such transmissions are used in forklift trucks and like vehicles.